Glucose in the cells:
Glucose provides the cells with fuel.
Glucose is obtained from the food we eat or
from glycogen which is the form that
glucose takes when it’s stored in the
liver. If glucose or glycogen are not
available, the cells look to stored fats
and protein for fuel. But one glucose
molecule can provide enough energy to make
36 ATP molecules. There are 4 steps
involved:
*glycolysis
*the preparatory step
*the citric acid cycle
*the electron transport system
GLYCOLYSIS – this step occurs in the cell’s
cytoplasm, not in the mitochondria. Glucose
is a 6-carbon molecule C6H12O6
During glycolysis, glucose is broken down
into 2 three-carbon pyruvate molecules. At
the end of glycolysis, the net gain of
energy is 2 molecules of ATP.
PREP STEP – this step prepares the
molecules for the citric acid cycle. So,
the pyruvate molecules enter the
mitochondria. In this step, each pyruvate
molecule forms a two-carbon acetyl group
and a carbon dioxide waste molecule. Each
acetyl group joins with another coenzyme to
form acetyl CoA, which takes the acetyl
group to the next step, which is the citric
acid cycle.
CITRIC ACID CYCLE – this is also called the
KREBS CYCLE. In this step, each acetyl
group is broken apart to form carbon
dioxide waste and energy in the form of
hydrogen ions. At the end of the citric
acid cycle, there is a gain of 4 ATPs. A
series of eight sequential steps occur in
the citric acid cycle. In the first step,
acetyl CoA from the Prep Step combines with
a 4-carbon fragment to form CITRIC ACID,
for which the cycle is named. Citric acid
is then the substance (or starting
molecule) for the remaining 7 reactions.
Each reaction is run by a different enzyme.
By the end of this cycle, glucose has been
broken down to form carbon dioxide waster
and high energy hydrogen ions and
electrons. These hydrogen ions and
electrons are carried by 3 molecules of
NADH and a molecule of FADH2. These
molecules are important in the final step
to produce ATP.
NADH = high energy molecule nicotinamide
adenine
FADH2 = high energy molecule flavin adenine
ELECTRON TRANSPORT SYSTEM – in this step,
the NADH and FADH2 move to the inner
membrane of the mitochondria and release
their hydrogen ions and high-energy
electrons to a carrier protein of the
electron transport system. Energy is both
lost and gained and finally the carrier
protein takes the hydrogen ions from the
inner membrane of the mitochondria to the
outer membrane. Now there becomes a higher
concentration of hydrogen ions in the outer
membrane than in the inner membrane. The
hydrogen ions try to diffuse back to the
inside membrane of the mitochondria but the
hydrogen ions can only diffuse through
special channels which help produce ATP
from ADP and inorganic phosphate (Pi).
The process of producing ATP from ADP and
Pi by using energy from transferred
electrons is called OXIDATIVE
PHOSPHORYLATION. The molecules of NAD and
FAD and ADP are low energy molecules
because they lack hydrogen and electrons.
However, they are recycled and used again
to keep the process going. This process is
efficient because NAD, FAD and ADP do not
have to be made again; they just recycle.
The net result of electron transport and
oxidative phosphorylation is about 34 ATPs.
So… what is CELLULAR RESPIRATION?
It is metabolism that uses oxygen and
produces carbon dioxide in order to make
ATP. Cellular respiration occurs in the
mitochondria. Remember that the first step
(glycolysis) did not occur in the
mitochondria and did not use oxygen. The
remaining 3 steps of the cycle are
considered to be cellular respiration. But
without glycolysis, the other steps would
not run or be efficient.
Once glycogen is depleted, the body looks
to fats and proteins to provide energy.
Here’s what happens… after we eat a meal,
the body has plenty of glucose, lipids and
amino acids to use for energy. The body
first uses available glucose. The excess
energy is stored as glycogen in the liver
and the rest is converted to fat and is
stored in fat tissue. Between meals, the
body then uses the stored glycogen, fats,
and sometimes protein for energy.
Fats actually carry more energy than
glucose. The body stores fats as
triglycerides, which can be broken down
into glycerol and fatty acids. Glycerol
can be converted either into glucose or
pyruvic acid, which will then become part
of the citric acid cycle. Fatty acids can
enter the citric acid cycle because they
break down into acetyl groups (in the Prep
Step). Because triglycerides have 16-18
carbon tails, they can make lots of ATP.
Proteins can produce as much energy as
glycogen. Proteins are broken into amino
acids. The carbon portion of the amino
acids enters the citric acid cycle. Even
though proteins can provide the body with
an energy source, proteins are used mostly
as enzymes and for structure. However,
proteins can be used by the body for energy
when the body is starving.
Finally we see that oxygen is necessary to
keep the citric acid cycle and the electron
transport chain going. These systems are
called aerobic metabolism because they use
oxygen. There is one system in the body
that does not use oxygen… that is
glycolysis, which we mentioned earlier.
Glycolysis is anaerobic respiration.
Without oxygen, glucose is broken down into
pyruvate, which is then converted into
lactic acid. When lactic acid builds up,
the body gets the burning sensation and
cramping due to muscle fatigue. This
happens because there is not enough oxygen
in the body. This process makes only 2ATPs
and it is not efficient.